1. Field of the Invention
This invention relates to graphical recording apparatus, and more particularly, to an electronic tachograph unit for use with a vehicle for recording the distance travelled by the vehicle, the speed of the vehicle, and similar data in relation to time.
2. Description of the Prior Art
Tachograph units are employed in the transportation industry to obtain graphical records which are useful in determining vehicle operating costs, operator performance and vehicle scheduling. In the trucking industry, for example, tachograph units are used to provide a chart record of vehicle speed and distance travelled in relation to time. The chart record also indicates idling time, lugging, loading and unloading times, stopping times and the like.
Commercially available tachograph units include a circular chart recorder, or a strip recorder, which provides a graphical representation of vehicle speed in units of miles (or kilometers) per hour (or engine speed in unit of revolutions per minute), and the distance travelled by the vehicle. The chart recorder is driven by a conventional clock mechanism which, in the case of a circular chart recorder, effects rotation of a paper chart relative to a speed stylus and an odometer stylus to permit recording of speed and distance information on the chart during a given time interval, typically a 24 hour period.
The speed stylus and the odometer stylus are coupled to the speed drive of the vehicle by a drive linkage which also drives a conventional speedometer and an odometer of the tachograph unit. Known tachograph units require complex mechanical linkages including correction gearing and lever assemblies to convert the output of the drive linkage, which is typically a flexible shaft which is rotated in direct relation to the speed of the vehicle or speed of the engine, to a form suitable for driving the recording and indicating apparatus of the tachograph unit. The flexible shaft and reduction gearing have limited life in the field and result in repetitive maintenance and a significant increase in vehicle down time.
A further consideration is that the input drive for tachograph units presently available is derived from a geared output on the vehicle such as an output from the vehicle engine transmission, drive shaft wheel and the like which is representative of the engine or vehicle speed. However, since the geared outputs may vary over a wide range, typically 500 to 1500 revolutions per mile, it is frequently necessary to tailor the input drive linkage for a given tachograph unit to a given vehicle by providing correctional mechanical gearing. Even with such compensation, when the vehicle is equipped with oversize or undersize tires, the unit drive will not provide an accurate representation of distance travelled, and erroneous recordings will be provided.
Accordingly, it would be desirable to have a tachograph unit which is readily adjustable for use with vehicles having different geared outputs. It would also be desirable to have a tachograph unit which does not require the use of a rotating flexible shaft or correctional mechanical gearing, thereby minimizing maintenance costs and vehicle and driver idle time.
In the field, anti-skid devices are commercially available from a number of manufacturers for use on vehicles, and provide an electrical output which is representative of the speed of the vehicle. While it would be desirable to use such output to drive a tachograph, it is apparent that the output signal provided by anti-skid devices of different manufacturers will vary, and the variation in wheel size and tire size of different vehicles is such that the nature of the output signal provided by the anti-skid device for a vehicle will vary significantly. It is desirable, therefore, to provide a tachograph unit which is conveniently adjustable to accept any of the outputs provided by the different units, and utilize such output to provide accurate speed and distance recording.